When an electromagnetic (EM) wave is incident upon an interface (or boundary) between two different types of materials the result is a reflected wave back into the primary material and a transmitted wave into the secondary material. This is true regardless of the materials as long as they are different. One special case is when it is important to contain the initial wave within the primary material by using a metal wall as the secondary material. The reflected wave is then nearly 100% of the incident energy and interacts with the incident wave to create “standing waves” or modes in the volume of the primary material. These modes are a varying energy profile of peaks and nulls, and this is true regardless of the polarization and incident angle of the incident wave.
A common example of this special case is in a microwave oven, used for cooking and heating of foods where the primary material is simply air and the secondary materials are the metal walls forming a cavity. For example, typical microwave ovens are designed with flat metal walls, the result of which are 3-dimensional modal patterns in the electric field, contributing to the uneven heating (cooking) of food. To smooth out the heating characteristics, a rotating turntable is commonly utilized to support the food so the cooking averages within the field due to moving the food. While this does provide better average heat distribution, there still is significant variation in the cooking.